Method for determining mud weight requirements from bulk density measurements of shale cuttings



Nov. 5, 1968 E. H DOREMUS 3,409,092

METHOD FOR DETERMINING MUD WEIGHT REQUIREMENTS FROM ULK DENSITYMEASUREMENTS OF SHALE CUTTINGS Flled Jan. 17, 1967 2 Sheets-Sheet 1 FIG.

WEIGHT-PPS INVENTOR EUGENE H. DOREMUS Nov. 5, 1968 E. H. DOREMUS3,409,092

METHOD FOR DETERMINING MUD WEIGHT REQUIREMENTS FROM BULK DENSITYMEASUREMENTS OF SHALE CUTTINGS Flled Jan. 17, 1967 2 Sheets-Sheet 2 FIG.2

Puma QZ wDOIh rhauo auuc osusnv 0F SHALE CUTTINGS INVENTOR EUGENE H.DOREMUS United States PatentO 3,409,092 METHOD FOR DETERMINING MUDWEIGHT RE- QUIREMEN TS FROM BULK DENSITY MEASURE- MENTS OF SHALECUTTINGS Eugene H. Doremus, Morgan City, La., assignor to Gulf OilCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan.17, 1967, Ser. No. 609,884 8 Claims. (Cl. 175-50) ABSTRACT OF THEDISCLOSURE A method of determining the changes in mud weight re quiredwhen drilling into or through abnormally high pressured shale whichcomprises taking bulk density measurements of the cuttings on site andrelating the changes in shale bulk density to the required changes inmud weight.

This invention relates to a method for determining mud weightrequirements from bulk density measurements of shale cuttings,especially with relation to drilling wells through regions of abnormallyhigh pore pressure shale.

In certain areas of the earth where oil is found, of which one exampleis the US. Coast of the Gulf of Mexico extending from the Rio GrandeRiver to the delta of the Mississippi River, there are subterraneanlayers of shale known variously as low resistivity shale, low densityshale, low sonic velocity shale, abnormal shale, or most generally,trouble shale. These shales are characterized by an abnormally highpressure of fluids within their pores. These high pressures are thoughtto have been caused by abnormalities that occurred during the formationand compaction of the layers which resulted in the fluid in the layerssupporting an excessi've part of the overburden weight. The fluids maybe natural gas, oil, or salt or fresh water. When drilling a wellthrough such a high pressure shale many problems are encountered, themost serious of which is the possibility of a blow-out. Other problemsinclude salt water flows, mud contamination, and stuck drill pipe. Ablow-out is an accident wherein the high pressure encountered within theformation literally blows the entire column of drilling mud out of thehole. If the driving force is petroleum or gas or a combination ofthese, a fire often results which is destructive of equipment and evenhuman life. Even if no fire results, a blowout is very expensive interms of the damaged or destroyed drilling rig and other equipment, andpossible harm to the drillers, as well as the possibility of the wellcaving in which results in a lost well.

The presence of a region of high pressure shale evidences itself bygas-kicks, salt water flows, change in drilling rate, and formationsloughing which may result in stuck pipe.

The present invention provides a method of predicting the requiredincrease in mud weight or density as the well penetrates the troubleshale, and before the penetration in said region is so deep that it istoo late to correct the mud weight. With prior methods it was necessaryto run a log of the well, which required that the well be drilled. Withthe present invention, the high pressure shale is identified as the wellis being drilled, which helps the driller both in preparing to deal withthe trouble shale, and also aids in selecting casing points. Also,knowing the pressure-depth relationship of the well aids generally, forexample, in reservoir evaluations, drilling, and completion. Anassociated problem is dealing with the return to normally pressure shaleafter the well has passed through the layer of trouble shale. If thisreturn is not 3,409,092 Patented Nov. 5, 1968 ice recognized, theincreased mud weight, needed to safely drill the high pressure shale,could cause lost circulation in the normal shale. The invention alsoprovides for the recognition of the return to normally pressured shaleto inform the driller to take appropriate action. Also, by decreasingmud weight, drilling rate can be increased.

Some practices used in drilling high pressure shale, which are replacedin whole or in part by the method of the present invention are asfollows:

When a gas-kick is first encountered mud weight is increased until thegas stops kicking. This method is eifective, but has severaldisadvantages including the fact that the gas-kick is not encountereduntil the hole is into the high pressure shale, and may be so deep intoit that it may be too late to prevent a blow-out. Also it is necessaryto overincrease the mud weight which is expensive in terms of the costof the mud additives, and also could cause a fracture up-hole from theregion of the high pressure shale. Another disadvantage is that it isnecessary to stop drilling and lift the drill pipe off the bottom of thehole to permit free circulation of the mud with the additives therein.

Another method entails running resistivity logs and observing a changein gradient toward lower values of resistivity to indicate the presenceof high pressure shales. The disadvantage here is that the hole is wellinto the high pressure shale before the well is logged.

A third method of handling high pressure shale is to reduce the rate ofdrilling. This suffers from the obvious disadvantage of the increasedcost of drilling the well.

With the method of the present invention, the driller is given earlywarning of the trouble shale, long before a gas kick would haveoccurred.

In the accompanying drawing forming a part of this disclosure: FIG. 1 isa graphical representation of changes in bulk density and required mudweights; and FIG. 2. is an example of a curve of the bulk densityreadings taken during drilling of a well.

It is known that the bulk density of normal shale increases with depthby a regular amount. High pressure shales are characterized by adecrease in bulk density with increased depth. By measuring the bulkdensity of the cuttings at the site, and by the use of certainrelationships relating the decrease in bulk density to mud weightrequirements, the weight of the mud can be increased before the hole hasgone very deep into the high pressure shale. If the well is to bedrilled through the high pressure shale, the method of the inventioninforms the driller of the return to normal pressure. One correctiveaction he can take would be to set casing through the trouble layer andcontinue to drill with reduced weight mud.

As used throughout this application, the term high pressure shale willbe understood to mean any shale or sand-shale sequence in which thepore-fluid pressure is greater than approximately 0.465 times the depthof the hole. The steps of the invention in detail are as follows:

The driller has the curve of FIG. 1 before he starts to drill the newwell. The curve shown in FIG. 1 was prepared from data obtained from thedrilling of a number of wells in Southern Louisiana. The curve of FIG. 1can be used as a starting point when drilling in high pressure shales inother areas, it being understood that it may be necessary to slightlymodify the curve depending upon the peculiarities of the other area. Themanner of doing this will become apparent in the following discussion.

There are several ways in which a curve like the curve of FIG. 1 can beobtained and which can also be utilized to obtain data to adjust ormodify the curve of FIG. 1 for a region having high pressure shalecharacteristics dif- 3. ferentfrom those found in the Southern Louisianaarea.

One way this can be done is to directly generate the curve from datataken during drilling of a well. In this case, the driller wouldovercome'gas kicks, water flows, and the like by simply increasing mudweight until the problem is alleviated. If data is kept on theseincreases in mud weight and measurements of bulk density are taken whilethis is done, then a curve like the curve of FIG. 1 can be generateddirectly, or that data can be used to modify the curve of FIG. 1. Itshould be noted however, that the above outlined procedure does notconstitute the method of the invention since no attempt is made to usethe data to predict mud weight requirements for the well being drilled.Further, the method outlined above is not usually performed on the site,the cuttings being transmitted to a laboratory for purposes of takingbulk density measurements. This time delay would be intolerable topredict mud weights.

Another way of generating a curve like the curve of FIG. 1, or obtainingdata to check or modify the curve of FIG. 1, is by the use of other logsrun in wells adjacent the site of the new well, where it is expectedthat the new well will traverse the same or similar high pressure shaleencountered by the existing well. In such cases, there is usually nobulk density data available for the existing well. One of the factorscontrolling the density of shale is the variation in fluid content for aunit volume of shale; an increase in the percentage of fluid, usuallysalt water results in a decrease in bulk density and vice versa. Theincreased fluid content also effects other parameters: It causes anincreased conductivity and a decreased sonic velocity. There issubstantial correlation betweenthe degree of variation of each of thesethree parameters, density, sonic velocity, and conductivity, and changesin the fluid content of shale. It has been found that there is asubstantially direct correlation between abnormal changes inconductivity or sonic velocity encountered in the high pressure'shaleand changes in the bulk density in the high pressure shale. A curverelating conductivity or sonic velocity to depth for the existing wellis obtainable from logs of the existing well. By measuring the changesin these parameters for the existing well, and using the known values ofmud weight which were used in drilling the existing well, and byrelating the changes in these parameters to known values of bulk densityfor the normal shale, a curve like the curve of FIG. 1 can be generated.

In cases where formation pressure measurements have been made in theexisting well, then that data can be used by relating it to sonicvelocity or conductivity logs for that well, and proceeding as above.

In the case where a first well is drilled in a new area where there areno other wells, or in the case Where it is anticipated that the adjacentwells did not encounter the formations it is anticipated the new Wellwill encounter, then the simplest procedure would be for the driller touse the curve of FIG. 1 as a first approximation and modify it as hedrills by means of getting new points during drilling. Such new pointswould be obtained from the required increases in mud weight necessary toovercome gas kicks, etc., and related to measured changes in measuredbulk density.

Exploration information such as seismic surveys and gravity surveys canbe helpful. Seismic surveys can help locate abnormally pressured shalesbecause these zones are often diapiric intrusions and as such do notproduce any reflections. The lack of reflections could indicate othersituations, but they at least alert the driller to the possibility ofthe existence of trouble shale. The low density of high pressure shalecould appear on a gravity survey as an anomalous low gravity area. Thiswould also alert the driller to the possibility of the existence oftrouble shale.

Once supplied with the curve of FIG. 1, the driller proceeds to drillthe new well, and periodically takes bulk density measurementsofthe-cuttings produced. As the.

well traverses the normal shale, a trendline, indicated by the bracketedregion 10 on FIG. 2, is established for the new well. At some point,which is the beginning of the high pressure shale, the bulk density ofthe cuttings decreases. The trend line is extended, as indicated bydotted line portion 12. As soon as the trend line'is established, or itis clear that the well is beginning to enter the'high pressure shale,measurements are taken from the curve of FIG. 2 from the actual pointrepresentative of bulk density over to the leftto extended portion 12,and this measurement indicates the difference in bulk density. With thisdifference the driller refers to the curve of FIG. 1 which directlygives him the mud weight increase required for that much-of a changeinbulk density. The use of two curves is by way of example only. Anyequivalent technique, such as transparent overlays or the like, areincluded in the scope of the invention. I

There are several procedures which must be followed in the actual takingof the bulk density of the cuttings. The cuttings collected from theshale shaker must be thoroughly washed until free of all kneaded,swelled, or otherwise soft shale and/or mud and until only firm shaleremains. Care must be exercised in this step because if the shale ishandled excessively While being washed, some of the firm shale willbecome soft, and in fact, if this is carried to an extreme all of thecuttings can be reduced to soft unusable shale. The washed, firm shalecuttings must be drained and then surface dried. Rolling or blottingwith paper towels has been found satisfactory.

It is important that the density measurements be made as soon aspossible after the washing and drying to avoid any internal drying ordecomposition of the shale which would destroy meaningful conclusions.An important advantage of the method of the invention is that it can becarried out at the drilling site. When samples are packed and carried toa laboratory off the drilling site, it has been found that the cuttingsdeteriorate by air dryin g or taking on excess water, or in other ways.

Any suitable method can be used to actually take the bulk densitymeasurement of the cuttings. The pycnometer bottle method involvesweighing the sample with and Without a known specific gravity liquid andthen calculating the bulk density by known formulae.

The density gradient liquid column method involves a mixture of twomiscible liquids of different specific gravities to produce a blendedliquid of gradiated specific gravity. This mixture is put in a graduatedcylinder to give a straight line gradient of specific gravity which maybe calibrated by use of beads of known specific gravity. The samples arethen allowed to sink into the column until they stop at some level whichlevel directly gives their bulk density.

The Jolly balance is especially designed to determine the specificgravity of solids. The weight of the samplein air and the weight of thesample in Water are quickly determined with the instrument. The apparentloss of weight in water is the weight of the bulk of water displaced bythe bulk of the sample. The weight of the sample in air is divided bythe weight of the equal bulk of Water.

The Le Chatelier specific gravity bottle method is not satisfactory foron-site work because of the problem of cleaning the relatively smallnecked bottles that are used. Another method utilizes a series ofbottles filled with incrementally increased specific gravity liquidsthrough the desired range. The cuttings are dropped into the bottlesuntil one bottle is found where the cutting just begins to float.

The invention encompasses any method or technique for determining thebulk density.

Various other characteristics of high pressure shale have beeninvestigated to find whether or not they can be utilized in the methodof the invention to predict mud weight requirements. The mineralogicalanalysis of cut tings in some wells do show a change in mineralassemblage associated with the high pressure shales. However, thesedifferences are slight, cannot be readily obtained on site, and do nothave any particular correlation to formation pressure.

Faunal assemblages can be used in some cases to delineate the highpressure shale interval, but like the mineral assemblage, do not have acorrelation to formation pressure and therefore cannot be used topredict mud weight requirements.

Other shale properties, such as cation exchange capacity and graindensity, cannot be used to distinguish normal pressure from highpressure shale.

0f the three parameters that are used to estimate formation pressures,density, sonic velocity, and conductivity, shale density has theadvantage, for reasons which include that it is the simplest to measurewhile drilling is in progress. It offers improvementspover presenttrouble indicators in that it not only gives early warning of anabnormally high pressure zone and offers means to determine the degreeof pressure increase; but will also indicate when the pressure returnsto normal.

While the invention has been described in some detail above, it is to beunderstood that this detailed description is by way of example only, andthe protection granted is to be limited only within the spirit of theinvention and the scope of the following claims.

I claim:

1. A method of determining mud weight requirements when drilling a wellthrough high pressure shale which comprises the steps of preparing afirst graphical relationship of mud weight required and changes in bulkdenof the shale difierent from normal changes in bulk density, takingbulk density measurements of the cuttings as the well is drilled, usingsaid bulk density measurements to make a second graphical representationof the normal increase in bulk density with depth to establish a normaltrend line of the normal increase of bulk density, extending said normaltrend line through the region of high pressure shale on said secondgraphical representation, and using the difference in bulk densitybetween cuttings taken from the high pressure shale and the bulk densitypredicted by said extended portion of said normal trend 6 line todetermine the mud weight required from said first graphicalrelationship.

2. The method of claim 1, wherein the bulk density of the shale cuttingsis determined by the gradient liquid column method.

3. The method of claim 1, wherein the bulk density of the shale cuttingsis determined by the pycnometer bottle method.

4. The method of claim 1, wherein the bulk density of the shale cuttingsis determined by the Jolly balance.

5. The method of claim 1, wherein the shale cuttings are washed andsurface dried before the bulk density measurements are made.

6. The method of claim 1, wherein said first graphical relationship isdetermined from conductivity logs of wells adjacent the Well beingdrilled.

7. The method of claim 1, wherein said first graphical relationship isdetermined from sonic logs of wells adjacent the well being drilled.

8. The method of claim 1, wherein said first graphical relationship isdirectly generated by drilling an experimental well, taking bulk densitymeasurements of the cuttings, increasing mud weight until any evidencesof high pressure shale are overcome, and directly generating said firstgraphical relationship from the records of increases in mud weight anddecreases in bulk density made during drilling of said experimentalwell.

References Cited UNITED STATES PATENTS 2,214,674 9/1940 Hayward 73-1532,302,996 11/1942 Lilligren 73-153 X 3,368,400 2/1968 Jorden et al 50 XOTHER REFERENCES Wallace, William E., Jr., Will Induction Log YieldPressure Data? In Oil & Gas Journal, 62 (37), Sept. 14, 1964, pp.124-126. TN 860.039.

STEPHEN J. NOVOSAD, Primary Examiner. I. A. CALVERTS, AssistantExaminer.

